Integrated circuits (ICs) are cornerstones of myriad computational systems, such as personal computers and communications networks. Purchasers of such systems have come to expect significant improvements in speed performance over time. The demand for speed encourages system designers to select ICs with superior speed performance. This leads IC manufacturers to carefully test the speed performance of their designs.
FIG. 1 depicts a conventional test configuration 100 for determining the signal propagation delay of a test circuit 110 in a conventional IC 115. A tester 120 includes an output lead 125 connected to an input pin 130 of IC 115. Tester 120 also includes an input line 135 connected to an output pin 140 of IC 115.
Tester 120 applies an input signal to input pin 130 and measures how long the signal takes to propagate through test circuit 110 to output pin 140. The resulting delay is the timing parameter for the path of interest. Such parameters are typically published in literature associated with particular ICs or used to model the speed performance of circuit designs that employ the path of interest.
Conventional test procedures are problematic for at least two reasons. First, many signal paths within a given IC cannot be measured directly, leading to some speculation as to their true timing characteristics. Second, testers have tolerances that can have a significant impact on some measurements, particularly when the signal propagation time of interest is short. For example, if the tester is accurate to one nanosecond and the propagation delay of interest is measured to be one nanosecond, the actual propagation delay might be any time between zero and two nanoseconds. In such a case the IC manufacturer would have to list the timing parameter as two nanoseconds, the worst-case scenario. If listed timing parameters exceed worst-case values, some designs may fail. Thus, IC manufacturers tend to add relatively large margins of error, or "guard bands," to ensure that their circuits will perform as advertised. Unfortunately, this means that those manufacturers will not be able to guarantee their full speed performance, which could cost them customers in an industry where speed performance is paramount.
Programmable logic devices (PLDs) are a well-known type of digital integrated circuit that may be programmed by a user (e.g., a system designer) to perform specified logic functions. One type of PLD, the field-programmable gate array (FPGA), typically includes an array of configurable logic blocks, or CLBs, that are programmably interconnected to each other and to programmable input/output blocks (IOBs). This collection of configurable logic may be customized by loading configuration data into internal configuration memory cells that, by determining the states of various programming points, define how the CLBS, interconnections, and IOBs are configured.
Each programming point, CLB, interconnection line, and IOB introduces some delay into a signal path. The many potential combinations of these and other delay-inducing elements make timing predictions particularly difficult. FPGA designers use circuit models, called "speed files," that include delay values or resistance and capacitance values for the various delay-inducing elements that can be combined to form desired signal paths. These circuit models are then used to predict circuit timing for selected FPGA configurations.
Manufacturers of ICs, including FPGAs, would like to guarantee the fastest timing specifications possible without having any circuits fail to meet timing specifications. More accurate measurements of circuit timing allow IC manufacturers to use smaller guard bands for ensuring correct device performance, and therefore to advertise higher speed performance. There is therefore a need for a more accurate means of characterizing IC speed performance.
In U.S. Pat. No. 5,790,479, Robert O. Conn describes a ring oscillator circuit formed in an FPGA, and a counter for counting the number of oscillations that occur for a signal propagating repeatedly around the ring during a specified period of time. The oscillator allows a fast signal to propagate many times along a path and thus increases the accuracy with which the path delay can be measured. Information in U.S. Pat. No. 5,790,479 is incorporated herein by reference. However, this patent does not distinguish between the delay of a rising edge propagating along the path and the delay of a falling edge propagating along the path. Since the tightest guarantees can be made when these two delays are separately known, it is desirable to be able to measure rising and falling edge delays separately.